A grain oriented electrical steel sheet is mainly utilized as an iron core of a transformer and required to exhibit superior magnetization characteristics, e.g., low iron loss in particular.
In this regard, it is important to highly accumulate secondary recrystallized grains of a steel sheet in (110)[001] orientation, i.e., what is called “Goss orientation,” and to reduce impurities in a product steel sheet. However, there are restrictions on controlling crystal grain orientations and reducing impurities in view of production cost. Accordingly, there has been developed a technique of introducing non-uniform strain or grooves into a surface of a steel sheet by physical or chemical means to subdivide the width of magnetic domains to reduce iron loss, i.e., magnetic domain refinement technique.
For example, JP 57-2252 B proposes irradiating a steel sheet as a finished product with a laser to introduce high-dislocation density regions into a surface layer of the steel sheet, thereby narrowing the magnetic domain width and reducing iron loss of the steel sheet.
Further, JP 62-53579 B proposes forming grooves exceeding 5 μm in depth in a base steel of a final-annealed electrical steel sheet, under a load of from 882 MPa to 2,156 MPa (from 90 kgf/mm2 to 220 kgf/mm2), which is then heat treated at a temperature of 750° C. or higher, to thereby refine magnetic domains.
JP 3-69968 B proposes introducing linear notches (grooves) in a direction substantially perpendicular to the rolling direction of the steel sheet at intervals of at least 1 mm in the rolling direction, the notches each being 30 μm or more and 300 μm or less in width and 10 μm or more and 70 μm or less in depth.
The development of the aforementioned magnetic domain refining technologies has made it possible to obtain a grain oriented electrical steel sheet having good iron loss properties.
On the other hand, a grain oriented electrical steel sheet is applied with a tension coating mainly composed of silica and phosphate. The tension coating causes a tensile stress in the grain oriented electrical steel sheet, to thereby effecting improvement in the magnetostrictive property and reduction of transformer noise.
For example, JP 3651213 B, JP 48-39338 A, and JP 50-79442 A each propose a tension coating obtained by applying a treatment solution containing colloidal silica, phosphate, and one or at least two selected from a group consisting of chromic anhydride, chromic acid, and dichromic acid, and baking the solution thus applied.
Further, as an example of a tension coating for a grain oriented electrical steel sheet containing, as main components, colloidal silica and phosphate while being free of chromic anhydride, chromic acid, and dichromic acid, JP 57-9631 B discloses an insulating top coating layer containing colloidal silica, aluminum phosphate, boric acid, and one or at least two selected from a group consisting of sulfates of Mg, Al, Fe, Co, Ni, and Zn. Further, JP 58-44744 B discloses a method of forming an insulation film containing colloidal silica, magnesium phosphate, and one or at least two selected from a group consisting of sulfates of Mg, Al, Mn, and Zn, without containing chromium oxide.
In the meantime, a grain oriented electrical steel sheet obtained as a final product is cut by a shearing machine into electrical steel sheets each having a predetermined length and shape. Then, the electrical steel sheets thus cut are stacked, to thereby serve as an iron core of a transformer. Very high precision is required for the cutting length in the cutting of a steel sheet by the shearing machine. For this reason, it is necessary to dispose a roll called a measuring roll on the front side of the shearing machine to come into contact with a steel sheet and measure the length of the steel sheet through the rotation of the roll, to thereby define the cutting position for the shearing machine.
We discovered that the aforementioned provision of magnetic domain refining treatment through the formation of grooves has the following problem. That is, as illustrated in FIG. 1, when pressed as rolled by a measuring roll R, a groove 1 is likely to develop plastic deformation in edges (corners) 10 where stress is concentrated, thereby causing an increase in transformer noise.
It could therefore be helpful to provide a grain oriented electrical steel sheet having excellent noise property capable of suppressing noise of an actual transformer which is configured by a steel sheet material having grooves formed therein for magnetic domain refining.